1. Field of the Invention
The present invention relates to a solid-state image sensor.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2012-019057 describes the relative disposition of a photoelectric converter and a readout circuit in a solid-state image sensor which forms one imaging area by arranging a plurality of imaging blocks. FIG. 2 of Japanese Patent Laid-Open No. 2012-019057 describes an arrangement in which pixels in an odd numbered column and pixels in an even numbered column have a line-symmetric layout with respect to a symmetric axis between the odd numbered column and the even numbered column.
FIG. 1 shows an example in which two adjacent pixels are line-symmetrically disposed with respect to a line along a column direction as a symmetric axis. Each pixel complies with the layout of one pixel described in FIG. 6 of Japanese Patent Laid-Open No. 2012-019057. A current flows from a node n1 to GND through a transistor 303 connected to a photoelectric converter 202 via a node CVC and a transistor 304 to which an EN signal is connected. This current flows from left to right in the left-side pixel, and from right to left in the right-side pixel.
Generally, a process for forming a transistor includes an ion implantation step. There are, for example, ion implantation for forming a well, ion implantation for forming a source/drain region, ion implantation for adjusting a threshold, and the like. At that time, in order to prevent a channeling phenomenon, ion implantation (to be referred to as oblique ion implantation) can be performed without implanting ions in parallel to a normal on a semiconductor substrate surface in a direction with an inclination of about 7° with respect to the normal.
When oblique ion implantation is performed in the layout as shown in FIG. 1, since the current flowing direction in the left-side pixel differs from that in the right-side pixel, the relationship between an ion implantation angle and the current direction in the left-side pixel differs from that in the right-side pixel. Then, a voltage-current characteristic of the transistor slightly shifts between the left-side pixel and the right-side pixel, and a pixel output shifts between an odd numbered column and an even numbered column. This generates fixed-pattern noise.